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The alleged incident is believed to have taken place in a hotel in
The incident is believed to have happened after Barton became involved in an altercation with a 15-year-old Everton fan at the team's hotel.
Barton is believed to have reacted to comments made by the teenager with team-mate Richard Dunne stepping in to act as peacemaker.
A club spokesman confirmed: "We are investigating an incident which took place in the hotel bar in the early hours of Friday which primarily involved Joey Barton. Our enquiries are continuing into the event."
City are now investigating the incident and are set to discipline Barton.
Eastlands boss Stuart Pearce has laid the blame for the incident on Barton and the fight may cast doubt over the midfielder's future at the club.
It is not the first time Barton has found himself in trouble after he was handed a six-week fine for an incident at the club's Christmas party last year.